⚗️ Aromatic Compounds
Hückel's rule: 4n+2 pi electrons = aromatic
Aromaticity — Hückel's Rule
Aromatic compounds must be: cyclic, planar, fully conjugated, and have 4n+2 pi electrons (n=0,1,2...). 6 electrons (benzene), 10, 14, 18... Antiaromatic = 4n pi electrons. Nonaromatic = not cyclic/conjugated.
⚗️ Aromatic Compounds
Ortho/para directors = EDG (activate ring), Meta directors = EWG (deactivate ring)
EAS Directing Effects
Electron Donating Groups (OH, NH2, alkyl, halogens*): direct ortho/para, activate ring. Electron Withdrawing Groups (NO2, CN, COOH, CHO): direct meta, deactivate ring. *Halogens are o/p directors but deactivate.
⚗️ Aromatic Compounds
EAS mechanism: electrophile attacks → arenium ion → deprotonation restores aromaticity
EAS Mechanism
Step 1: Electrophile (E+) attacks pi system → forms arenium ion (carbocation, loses aromaticity). Step 2: Base removes proton → restores aromaticity. Aromaticity drives the reaction to completion.
⚗️ Aromatic Compounds
Nitration: HNO3 + H2SO4 → NO2+. Sulfonation: SO3/H2SO4. Halogenation: X2 + Lewis acid
Common EAS Reactions
Nitration electrophile = NO2+ (nitronium ion). Sulfonation = SO3. Halogenation needs Lewis acid catalyst (FeBr3, AlCl3). Friedel-Crafts alkylation/acylation = carbocation or acylium ion electrophile.
⚗️ Aromatic Compounds
Birch reduction: Na/NH3(l) → 1,4-cyclohexadiene (reduces non-substituted positions)
Birch Reduction
Dissolving metal reduction (Na or Li in liquid NH3 + alcohol). Reduces benzene ring to 1,4-cyclohexadiene. EDG groups: double bonds remain on substituted carbons. EWG groups: double bonds away from substituent.